1. Field of the Invention
The present invention relates to a fringe field switching mode LCD and, more particularly, to a fringe field switching mode LCD obtaining a maximum transmittance while using liquid crystal layers of positive dielectric anisotropy.
2. Description of the Related Art
An IPS (In-Plane Switching) mode LCD has been proposed in order to overcome low viewing angle of TN (Twisted Nematic) mode LCD. The IPS mode LCD has a structure that an upper and a lower substrates are arranged opposite to each other with a predetermined distance. A liquid crystal layer comprising a plurality of liquid crystal molecules is interposed between the substrates.
Here, pixel and counter electrodes, comprising opaque materials, are formed on the lower substrate to drive liquid crystal molecules and separated with a distance longer than that between the upper and lower substrates to form a parallel electric field. And also the pixel and counter electrodes have relatively large width to maintain a constant electric intensity. And, horizontal alignment layers are respectively interposed between upper and lower substrates and liquid crystal layer.
This IPS mode LCD has an improved viewing angle since liquid crystal molecules are arranged in parallel with a substrate. However, it has a disadvantage of low transmittance.
In order to overcome the low transmittance of IPS mode LCD, a fringe field switching mode LCD (hereinafter referred to as FFS-LCD) has been proposed. The FFS-LCD has pixel and counter electrodes made from transparent conductors and the distance between the electrodes are narrower than the distance between the upper and lower substrates to form a fringe field on the electrodes.
The conventional fringe field switching mode LCD will be described in detail with reference to FIG. 1.
FIG. 1 is a cross-sectional view of conventional fringe field switching mode LCD.
Referring to FIG. 1, a lower substrate 1 and an upper substrate 10 are arranged opposite to each other with a predetermined distance (d: hereinafter referred to as cell gap).
And, a liquid crystal layer 15 is interposed between the lower substrate 1 and the upper substrate 10. The liquid crystal layer 15 comprises a plurality of liquid crystal molecules of positive or negative dielectric anisotropy.
A retardation, that is a result of multiplying cell gap d and refractive anisotropy xcex94n, is preferably 0.25 to 0.35 xcexcm to obtain maximum transmittance.
Although it is not shown in the drawings, a gate bus line and a data bus line are crossed on the lower substrate 1 to define a unit pixel and a thin film transistor (not shown) is disposed at the intersection of the lines.
And, a counter electrode 3 is formed in the unit pixel of the lower substrate 1. The counter electrode 3 comprises a transparent ITO (Indium Tin Oxide) layer and has a shape of slant or a plate.
A gate insulating layer 4 is formed on the upper part of the counter electrode 3 in a slant shape to overlap with the counter electrode 3. The distance between the counter electrode 3 and the pixel electrode 5 is narrower than cell gap d.
And, a horizontal alignment layer 6 is formed on the surface of the resulting lower substrate 1 to control initial arrangement of liquid crystal molecules. The horizontal alignment layer 6 has a rubbing axis to a predetermined direction and a predetermined pretilt angle.
On the other hand, a color filter 12 is formed on an upper substrate 10 opposite to the lower substrate 1. A horizontal alignment layer 14 is also formed on the surface of the color filter 12 to control initial arrangement of liquid crystal molecules. The horizontal alignment layer 14 also has a predetermined pretilt angle and a rubbing axis forming an angle of 180xc2x0 with that on the lower substrate.
The conventional FFS-LCD operates in a following method. First, when voltage differences are generated between a counter electrode 3 and pixel electrode 5, a fringe field is formed since the distance between the electrodes is narrower than cell gap d. The fringe field has influence on the upper parts of the counter electrode 3 and of pixel electrode 5 since the counter electrode 3 has a narrow open space between pixel electrodes 5, thereby driving most of liquid crystal molecules in a unit pixel. As a result, transmittance and aperture ratio are improved.
Generally, a conventional FFS-LCD employs liquid crystals of both positive and negative dielectric anisotropy as a liquid crystal layer. However, liquid crystals of negative dielectric anisotropy are preferred due to rapid response speed. And, the value of retardation is determined to obtain a maximum transmittance when using liquid crystals of negative dielectric anisotropy. Accordingly, when liquid crystals of positive dielectric anisotropy are used as the liquid crystal layer, it is difficult to obtain a maximum transmittance since the retardation conditions are not sufficient.
Therefore, the present invention has been made in order to solve the above-mentioned problems in the prior art. An object of the present invention is to provide a FFS-LCD to obtain a maximum transmittance while using liquid crystals of positive dielectric anisotropy.
In order to achieve the above object, the present invention comprises:
an upper and a lower substrates opposed to each other with a predetermined distance; a liquid crystal layer interposed between the substrates, comprising liquid crystal molecules of positive dielectric anisotropy, wherein a retardation of the liquid crystal layer is 0.3 to 0.5 xcexcm; a counter electrode disposed on the inner surface of the lower substrate; a pixel electrode disposed on the inner surface of the lower substrate, forming a fringe field with the counter electrode to drive liquid crystal molecules; and a horizontal alignment layer interposed between the upper and the lower substrates and liquid crystal layer, having a predetermined rubbing axis.